1. Field of the Invention
The present invention relates to a technology for preventing quality degradation of a liquid crystal display device, which results from a cell gap.
2. Description of the Related Art
As the information technology has developed tremendously in recent years, commercialization of a liquid crystal display device that is capable of high-definition display and is compact, lightweight and low power consumption has been desired also in the field of a portable terminal. As a liquid crystal display device meeting such a requirement, a reflective liquid crystal display device performing display by using external light as a light source is an odd-on favorite.
When being used outdoors during the daytime, the reflective liquid crystal display device can perform image display of which visibility is extremely high. Meanwhile, because the light source is not provided, it is difficult to brightly display an image in a dark place, and a use environment of the reflective liquid crystal display device is limited.
In this connection, a liquid crystal display device has been proposed, which includes a transmission area for transmitting light from the light source therethrough and using the light for the display, and a reflection area for reflecting the external light and using the external light for the display. This liquid crystal display device performs the image display by using the external light as the light source in the reflection area and using a backlight unit as the light source in the transmission area, thereby absorbing the limitations on the use environment.
Moreover, for example, in Japanese Patent Laid-Open Publication No. 2002-303863, a liquid crystal display device has been disclosed, which includes a semi-transmission area having a reflection portion and a transmission portion in each pixel, and a reflection area having only the reflection portion in each pixel. In this liquid crystal display device, the backlight unit is not required in the reflection area, and accordingly, power consumption can be reduced.
Meanwhile, a liquid crystal display device giving more importance to improvement of the visibility has also been proposed. Such a liquid crystal display device is an example that includes a semi-transmission area having both of a reflection portion and a transmission portion in each pixel, and a transmission area having only a transmission portion in each pixel.
According to the liquid crystal display device, the display areas are used properly depending on a type of image information such that a current time the like are displayed on the semi-transmission area and a picture image is displayed on the transmission area. In such a way, the visibility of the displayed image can be improved on the transmission area while absorbing the limitations on the use environment and reducing the power consumption.
FIG. 1 is a cross-sectional view showing a configuration of one pixel in a semi-transmission area of a conventional liquid crystal display device 1B. As shown in this drawing, this pixel includes a transmission portion 12A and a reflection portion 12B.
The liquid crystal display device 1B includes an array substrate 111 formed of transparent glass or the like as a material, plural signal lines and plural scan lines, which are not shown, are formed on the array substrate 111 so as to intersect each other, and a transparent insulating film 112 is further formed thereon. On the transparent insulating film 112, a transparent pixel electrode 113 formed of indium tin oxide (ITO) as a material is formed.
An opposite substrate 116 formed of transparent glass as a material is disposed so as to be opposite to the array substrate 111. On the opposite substrate 116, a color filter 117 corresponding to a color of the pixel is disposed. On the color filter layer, a transparent insulating film 118 is formed, and a transparent opposite electrode 119 formed of the ITO or the like as a material is formed thereon.
Between the array substrate 111 and the opposite substrate 116, a liquid crystal layer 114 formed of multi-domain vertical alignment (MVA) liquid crystal as a material is disposed.
In the reflection portion 12B, unevenness is formed on the transparent insulating film 112, and correspondingly thereto, unevenness is also formed on the pixel electrode 113. Moreover, on the pixel electrode 113, an opaque reflection electrode 113B for reflecting external light, which is formed of aluminum or the like as a material, is formed. Correspondingly to a shape of the pixel electrode 113, unevenness is also formed on the reflection electrode 113B.
Moreover, in order to perform the optimum display in the reflection portion 12B, a transparent bump layer 118B for narrowing a cell gap is formed on the transparent insulating film 118. Here, thickness T of the bump layer 118B is assumed to be approximately 3 μm. In a region where the bump layer 118B is formed, a columnar spacer 115B for constantly maintaining the cell gap is disposed.
A basic configuration of the above-described transparent area where each pixel has only the transmission portion is similar to that of the transmission portion 12A, but is different therefrom in including a spacer for constantly maintaining a distance between the substrates. Height H of the spacer is substantially equal to thickness of the liquid crystal layer 114 in the transmission portion 12A. Here, the height H of the spacer is assumed to be approximately 6 μm.
Owing to the bump layer 118B, the cell gap in the reflection portion 12B is made narrower than the cell gap in the transmission area and the transmission portion 12A by the thickness T of the bump layer 118B. Accordingly, height HB of the spacer 115B is lower than the height H of the spacer of the transmission area. Specifically, the height HB is set so as to establish a relationship: H≈T+HB. Here, the height HB is approximately 3 μm.
The height HB of the spacer 115B is lower than the height H of the spacer of the transmission area, and accordingly, the spacer 115B must be formed by a process different from a process for forming the spacer of the transmission area. Therefore, in the liquid crystal display device 1B, the number of manufacturing processes is increased, and there is a possibility to reduce yield.
Moreover, the cell gap of the reflection portion 12B is determined by two main elements, which are: the thickness of the bump layer 118B; and the height of the spacer 115B. Specifically, the cell gap of the reflection portion 12B is not determined by a single main element, and accordingly, precision of the cell gap in the semi-transmission area is poor, and variations in characteristics are large. In addition, owing to such factors, there is a possibility that display unevenness may occur on the overall display area.